Method of antenna tuning

ABSTRACT

A method and apparatus for adjusting the physical dimensions of an antenna radiating element to its electrically resonant length at a predetermined frequency by coupling a radiation sensitive device between a radio frequency energy source and a dummy load. A direct current meter is coupled to the radiation device to obtain a reading on the meter which is proportional to the radio frequency energy traveling to the load. The value read on the meter is noted and the source of radio frequency energy is switched to the antenna element which is to be tuned. The physical length of the antenna element is shortened or lengthened to bring the reading of the meter to the value obtained when the radio frequency energy was connected to the dummy load. At this point the antenna element has been tuned and its radiation resistance will equal that of the dummy load.

United States Patent Peters June 10, 1975 METHOD OF ANTENNA TUNINGPrimary Examiner- EM Lieberman [75] Inventor: Carl L Peters, vista,Calif Attorney, Agent, or Fzrm-R. S. Sciascia; Roy Miller;

Robert F. Beers [73] Assignee: The United States of America asrepresented by the Secretary of the Navy, Washington, DC. [57] ABSTRACT[22] Filed: Mar. 1, 1974 A method and apparatus for adjusting thephysical di- [21] Appl NO 447 175 mensions of an antenna radiatingelement to its electrically resonant length at a predetermined frequencyby coupling a radiation sensitive device between a [52] U.S. Cl. 343/703radio frequency energy source and a dummy load. A direct current meteris coupled to the radiation device 51 Int. Cl. GOlr to Obtain a readingon the meter which is proportional of Search to the radio frequencyenergy traveling to the load The value read on the meter is noted andthe source of radio frequency energy is switched to the antenna [56]References Cited element which is to be tuned. The physical length ofthe antenna element is shortened or lengthened to UNITED STATES PATENTSbring the reading of the meter to the value obtained 2,365,207 12/1944Moles 343 703 When the radio f q y energy was Connected to the 2,565,900W116}, dummy load At this point the antenna element has OTHERPUBLICATIONS Hints and Kinks for the Radio Amateur: published byAmerican Radio Relay League, 1968, pg. 110.

been tuned and its radiation resistance will equal that of the dummyload.

3 Claims, 1 Drawing Figure 15h l6 VARIABLE RADIATION FREQUENCY COUPLINGSWITCH GENERATOR DEVICE l7 DUMMY VOLTMETER LOAD PATENTEIJJUH 10 I975 3.8 89268 IN l2\ 5W 1e VARIABLE RADIATION FREQUENCY COUPLING SWITCHGENERATOR DEVICE |9 17 DUMMY VOLTMETER E LOAD METHOD OF ANTENNA TUNINGBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to an improved method and measurement system for quickly andaccurately tuning the physical length of radiating antenna elements to adesired resonant frequency.

2. Prior Art A fundamental form of an antenna is a single wire orradiator whose electrical length is equal to approximately half thetransmitting wavelength. To obtain the most efficient transfer of energyfrom the transmitting source to the antenna system requires that theantenna be resonant at the desired frequency of operation and that theload resistance equal the radiation resistance of the antenna. Since theelectrical input impedance of a dipole antenna at its resonant frequencyis resistive the antenna system should be matched to the resistive valueof the radio frequency generating source in order to obtain maximumperformance from the antenna. The radiation resistance of the antennacan generally be defined as that value of resistance which whensubstituted for the antenna will dissipate the same amount of power thatis radiated by the antenna. When the radio frequency transmittersupplies energy at the resonant frequency the maximum amount of energysupplied to the antenna will be radiated into space. I-Ialf waveantennas in free space have a certain characteristic radiationresistance but the actual value of radiation resistance of any givenantenna depends upon its configuration and the size and the proximityand character of nearby objects to the antenna. Because of these effectswhich detune the antenna, merely calculating the antennas resonantlength will not insure its most efficient operation and it is necessaryto use some measuring technique to determine the actual electricallength of antenna which will be resonant at the frequency of operation.

Prior art techniques used for adjusting antenna systems eitherphysically or electrically require some means either of measuring theinput impedance of the antenna or measuring the standing wave ratio.Normally various bridge measurement configurations have been used. Theseinstruments are necessarily expensive for an accurate unit and requiresome radio frequency power for their operation. Standing wavemeasurements made at the antenna input terminals also have been utilizedto determine if the radiation resistance of the antenna is equal to theimpedance of the radio frequency source and transmission line.Comparative readings are made of the ratio of direct to reflectedvoltage from the antenna. The antenna is adjusted to bring the indicatorreading on the standing wave measurement to maximum null or close tozero which indicates a minimum of reflected energy from the antenna. Atthis point the antenna is matched at the resonant frequency of the radiofrequency generator. Tuning antennas by this technique requires acertain amount of radio frequency power delivered to the antenna inorder that a satisfactory indication on the standing wave meter beobtained. Also it is necessary to switch the standing wave meter betweenforward and reflected energy which can be quite troublesome whenevertuning antennas designed to operate at relatively high frequencies.

SUMMARY OF THE INVENTION An object of the invention is to provide amethod and apparatus system for tuning the physical length of an antennasystem to its resonant length at a predetermined frequency.

This and objects herein after defined are met by the invention whichrelates to a method of tuning a radiating antenna element to itsresonant length in a minimum of time and with relatively inexpensiveequipment that is simple to operate and allows tuning of antennasoperating at relatively higher frequencies with little. time, troubleand effort. A variable radio frequency generator tunable in the range ofthe resonant frequency desired is coupled to a dummy load that has aresistance comparable to the theoretical radiation resistance of theantenna. A radiation responsive device is coupled to the radio frequencyfield for providing a direct current voltage output proportional to themagnitude of the field. This direct current voltage is coupled to anindicating instrument. The amount of direct current voltage on theindicating instrument is noted and the radio frequency energy isswitched to the antenna. The antenna element to be tuned is eithershortened or lengthened to bring the reading on the direct currentindicator back to the original value that was observed when the radiofrequency energy was connected to the dummy load. When this pointisreached the antenna radiation resistance will be the same as theresistance of the dummy load and the antenna element will be at itsresonant point.

BRIEF DESCRIPTION OF THE DRAWING The only drawing in the case is asimplified block diagram representation of an antenna tuning apparatusconstructed in accordance with the teachings of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Referring tothe only drawing in the case, which shows a blocked diagram of variouselectronic devices coupled together for tuning an antenna element. Blockradio frequency outputs. The output is coupled to the antenna element tobe tuned via radiation coupling device 12 and switch 15. The conductors17, 18 and 20 may be coaxial cables or other suitable means to conducthigh frequency energy at a low energy loss.

Block 12 represents a radiation energy device that is capable of beingcoupled to the radio frequency field. The radiation coupling device 12generates a direct current voltage proportional to strength of the radiofrequency field. This device may be of the type disclosed in US. Pat.No. 3,360,726. The device disclosed uses the effect of the radiofrequency field to produce a measurable direct current voltage output.The radio frequency current flowing through wire 18 and 19 generates aradio frequency field about these wires. A germanium diode positionedclosely adjacent a single wire will have induced therein, energy that isconverted into direct current by the germanium diode. This inductiveeffect is believed to be due to the electro static field (E- field) ofenergy surrounding the wire and coupled to the germanium diode. Voltagemeter leads coupled to either end of the germanium diode will give adirect current voltage indication on voltmeter 13. The magnitude ofvoltage will depend on the frequency of the frequency generator 11 andthe value of the radio frequency output energy.

. A switch is coupled between the antenna element 16, dummy load 14 andradiation coupling device 12. Switch 15 is a good quality radiofrequency switch, such as a coaxial switch since the conductors 17, 19and 20 are coaxial cables.

Dummy load 14 is preferably a resistive load that has low capacitive andinductive reactance. Load 14 is of such a value that it closely matchesthe radiation resistance to which the antenna will tune. Normally thisresistance would be in the range of fifty ohms for matching the outputimpedance of the transmitter whichis coupled to the antenna, but thisvalue may be changed to other desired values within limits of theresistance of the antenna element to be tuned.

In the tuning operation of the apparatus shown in the only figure of thedrawing, variable frequency generator is tuned to the desired resonantfrequency of the antenna element 16. Switch 15 is connected to theresistive dummy load '14. The output of generator 11 is increased untila'substantial voltage indication is indicated by direct currentvoltmeter 13. The radiation responsive device 12 by induction from theradio frequency energy present in the wire from the generator 11 gives avoltage indication proportional to the energy flowing'throughthe dummyload 14. Switch 15 is activated to connect antenna element 16 to theradio frequency energy source 11. The indication on the voltmeter isbrought to its original reading by increasing or decreasing the physicallength of antenna element 16. Shortening or lengthing the antennaelement 16 may be accomplished by various means known to those skilledin the antenna art. When the antenna element has been adjusted to causethe same reading on voltmeter 13 as obtained by the dummy load 14, theantenna, element is properly adjusted for that frequency.

ingmethod used by the present invention is extremely simple to set upand permits accurate tuning by allowing small incremental adjustments ofantenna element length. 1

Although the preferred embodiment has been described, it will beunderstood that within the purview of this invention various changes maybe made in the 4 form, details, proportion and arrangement of parts, thecombination thereof and made of operation, which generally statedconsist in a device capable of carrying out the objects set forth, asdisclosed and defined in the appended claims.

What is claimed is: v

1. An antenna tuning system for adjusting the physical dimensions of anantenna to an electrically resonant length at a predetermined frequencycomprising:

a frequency generator means for supplying a variable radio frequencyoutput in the range of frequencies at which the antenna will resonate;

meansresponsive to radio frequency radiation coupled to said generatoroutput for supplying a direct current voltage output in proportion tothe output of said radio frequency generator;

measuring means electrically coupled to said radiation responsive meansfor providing an indication of direct current voltage output;

load means electrically coupled to receive a radio frequency output fromsaid radio frequency radiation responsive means; and switch meanselectrically coupled to said radio frequency radiation responsive meansand operable for selectively switching radio frequency energy betweensaid antenna and said load means;

whereby the frequency generator is adjusted to give an output indicationon measuring means at the resonant frequency of the antenna for tuningthe antenna by comparing the indication on the indicating means as theswitch means is operated between the antenna and the dummy load.

2. The antenna tuning system of claim 1 wherein said radio frequencyradiation responsive means comprises:

a'means of inductive coupling to the radio frequency energy forobtaining a direct voltage output that is proportional to the radiofrequency energy.

3. A method of adjusting the physical dimension of an antenna radiationelement to its electrically resonant length at a predetermined frequencyconsisting of:

feeding a radio frequency signal to a substantially resistive load;inductively coupling said radio frequency signal to a germaniumsemiconductor diode for generating a direct current signal that isproportional to the magnitude of said radio frequency signal;

coupling said direct current signal to an indicator for determining themagnitude of said radio frequency signal;

switching said radio frequency signal from said resistive load to saidantenna; and

adjusting the physical length of said antenna element until the readingon said indicator isthe same as observed on said indicator when saidradio frequency signal was connected to said resistive load.

1. An antenna tuning system for adjusting the physical dimensions of anantenna to an electrically resonant length at a predetermined frequencycomprising: a frequency generator means for supplying a variable radiofrequency output in the range of frequencies at which the antenna willresonate; means responsive to radio frequency radiation coupled to saidgenerator output for supplying a direct current voltage output inproportion to the output of said radio frequency generator; measuringmeans electrically coupled to said radiation responsive means forproviding an indication of direct current voltage output; load meanselectrically coupled to receive a radio frequency output from said radiofrequency radiation responsive means; and switch means electricallycoupled to said radio frequency radiation responsive means and operablefor selectively switcHing radio frequency energy between said antennaand said load means; whereby the frequency generator is adjusted to givean output indication on measuring means at the resonant frequency of theantenna for tuning the antenna by comparing the indication on theindicating means as the switch means is operated between the antenna andthe dummy load.
 2. The antenna tuning system of claim 1 wherein saidradio frequency radiation responsive means comprises: a means ofinductive coupling to the radio frequency energy for obtaining a directvoltage output that is proportional to the radio frequency energy.
 3. Amethod of adjusting the physical dimension of an antenna radiationelement to its electrically resonant length at a predetermined frequencyconsisting of: feeding a radio frequency signal to a substantiallyresistive load; inductively coupling said radio frequency signal to agermanium semiconductor diode for generating a direct current signalthat is proportional to the magnitude of said radio frequency signal;coupling said direct current signal to an indicator for determining themagnitude of said radio frequency signal; switching said radio frequencysignal from said resistive load to said antenna; and adjusting thephysical length of said antenna element until the reading on saidindicator is the same as observed on said indicator when said radiofrequency signal was connected to said resistive load.